Variable resistance device



Sept. 29, 1959 B. F. FREEBERG VARIABLE RESISTANCE DEVICE Filed Feb. 21, 1958 INVENTOR.

United States Patent O VARIABLE RESISTANCE DEVICE Benjamin F. Freeberg, Lombard, Ill., assigner to Vapor Heating Corporation, Chicago, Ill., a corporation of Delaware Application February 21, 1958, Serial No. 716,705

2 Claims. (Cl. 338-27) The present invention relates to variable resistance devices of the liquid column type. More particularly, the present invention is concerned with an electrical resistance element coated on the interior bore of a capillary tube containing the liquid column. Portions of the resistance element are shorted out by the liquid column the level of which varies within the capillary in response to a change in a physical condition such as pressure, temperature or momentum. The resistance coating is a chemically-deposited solid solution of a metallic carbide in a metal the coeicient of expansion of which may be varied to correspond to the coecient of expansion of the material of the capillary tube, for example, glass tubes having different coeicients of expansion.

Such devices have uses as accelerometers, pressure sensors, thermostats and a variety of other uses. For illustrative purposes the device is shown herein as a conventional and relatively small thermostat of the type shown in the patent of William M. Smith et al., No. 2,596,825, issued May 13, 1952. One contemplated use of the device as illustrated is in association with a control device for varying the delivery of heated or unheated air into an enclosure.

Heretofore it has been known to provide hollow insulating elements with electrical resistance elements portions of which are shorted out by an electrically conductive liquid. One such element, shown and described in the patent of William H. Bristol, No. 776,491, issued December 6, 1904, is a line resistance wire centered in a hollow ceramic body where it may be shorted out by a mercury column. A subsequent patent, No. 2,566,369, issued September 4, 1951, to Henry M. Putman, describes a capillary tube having a metal lining of platinum, silver or tin which is contacted by an electrically conductive liquid.

Later devices employed a coating resistive material applied to the bore of a capillary tube. A patent issued to Robert F. Brown, No. 2,736,782, issued February 28, 1956, describes the use or a material, other than a metal, having a high electrical resistance. Another patent, No. 2,743,340, issued April 24, 1956, to Edward F. Zoltanski, describes the use of a metallic oxide coating. The application of these known resistance elements to a liquid column device designed to be highly sensitive to changes in physical state and also designed to withstand substantial variations in temperature and physical shock or vibration has posed many problems.

In order to make devices of this general type sensitive to changes of temperature, for example, it is desirable to minimize the bore diameter of the capillary tube. Thus minimized relatively small changes in the overall volume of the contained liquid, mercury or a mercury thallium alloy, result in substantial changes in column height. Bore diameters have also been reduced for the purpose of minimizing the likelihood of liquid column separation or splitting when the device is subjected to physical shock or vibration. In this latter respect it has been found that liquid columns contained within bores rang- Patented Sept. 29, 1959 ICC ing in diameter from .005 to .009 of an inch resist splitting under all conditions of shock and vibration to which the 'device is likely to be subjected.

Application of some of the known resistive elements to bores of such limited size so restricts the bore that the device does not function. Malfunction occurs in devices using other of the known resistive elements when the device is exposed to extreme variations in temperature or thermal shock. The failure in this case results from the difference in the coeicients of expansion of the resistive element and the capillary to which it is applied and is attributable to cracks which develop in the resistive element under these conditions and which may, in extreme cases, cause the resistive element to separate from the Wall of the bore, crumble and collapse. Physical shock may cause failures of the same or a similar nature. The result is an opening of the electrical circuit through the resistance element.

The present invention improves upon the prior devices by providing a chemically-deposited coacting or resistive element which is strong and dense; bonds well with the glass capillary tube which carries it; has a thermal coefcient of expansion corresponding to that of the glass of the capillary tube; and, has a thickness not in excess of two millionths (.000002) of one inch when applied. The coating is chemically stable in the presence of the liquid column and of the inert, pressurizing gas also contained within the capillary tube which gas cooperates with the small bore diameter to prevent splitting of the liquid column as discussed above. Another feature of the coating is that its thickness may be varied to vary the ohmic resistance per unit length. One additional feature is that the coelhcient of expansion of the coating may be varied, within certain adequate limits, to correspond to the coefcient of expansion of the particular material, glass, upon which it is being coated. This is accomplished by varying the proportion of the metallic carbide to the metal in the solid solution comprising the coating. The desired result is attainable, it is theorized, because the metals used have a lower coeficient of expansion than does the glass while the metallic carbide has a higher coefficient of expansion than the glass. A solid solution of the two may be made to match, in terms of thermal expansion, the particular composition of glass to which it is bonded. Thermal cycling of the coated tube between F. and +300 F. with no evidence of cracking or separation from the tube wall serves to substantiate the theory.

The improved device has suiicient current-carrying capacity that it may be connected directly into an appropriate system thereby eliminating the need for relays or electrical amplifying means to bolster its signal.

It is the object of the present invention to provide a variable resistance device, of the character described, having the characteristics discussed above.

A preferred form of the invention, as it may be applied to a liquid column thermostat, is shown in the accompanying single sheet of drawings in which:

Fig. 1 is a full view of a variable resistance thermostat, partly in section, constructed in accordance with the present invention;

Fig. 2 is an enlarged longitudinal sectional view of the device shown in Fig. l; and

Fig. 3 is a sectional view taken substantially on the line 3-3 of Fig. 2.

Referring now to the drawings: The variable resistance thermostat shown is constructed to respond to temperature changes, therefore it is intended to be exemplary only, since the principles of the invention can be embodied in other similar devices constructed to respond to inertia or pressure. Said thermostat includes, in its general organization, a capillary tube 10 having a central capillary bore 11 formed therein. The bore is hermetically closed or sealed, as at 12, at its upper end and communicates with an interior chamber 13.0f a reservoir 14 located at its lower end. The tube and reservoir are preferably formed of lglass of a character selected to fulfill the thermal requirements of the device.

The chamber 13 is lled with an electrically conductive liquid 15 such as mercury or a mercury thallium alloy which liquid extends from the chamberv into the bore 11V of the capillary tube. The height of the mercury within the tube is dependent upon the extent of expansion of the overall volume of mercury. The upper portion ofthe tube is pressurized with an inert gas such as argon, helium, krypton, neon or zenon.

The inner cylindrical surfaceV of a limited portion of the bore 11 is coated with an electrically conductive coating 16. The coating is chemically-deposited upon the Wall of the bore and is a solid solution o f a metallic carbide in a metal selected and applied so as to have a coecient of expansion corresponding to that of the glass of the tube. The coating, in effect, provides an internal conductive sleeve or liner within the bore of the capillary tube throughout the expected range of movement of the mercury column.

Preferred solid solutions of metallic carbides in metals for coating the bore include tungsten carbide in tungsten and molybdenum carbide in molybdenum. These preferred materials, in solid solution, have the desired thermal characteristics; are chemically stable in the presence of mercury 0r a mercury thallium alloy and of `an inert, pressurizing gas; and, have sufficient current-carrying capacity. In addition they may be chemically deposited along pre-determined portions of the bore with relative ease.

Two lead-in wires 17 and 18 are illustrated making electrical contact with upper and lower portions respectively of the electrically conductive coating 16, However, additional leads may be provided if, for example, it is desired to vary resistance to more than one electrical load or for other purposes. Various means may be employed for establishing the electrical connection between the llead-in wires and the coating. In the form shown, the coating 156 makes physical and electrical contact at its upper portion with a transverse'metallic conductor 19 which is embedded in and passes through the glass material of the tube substantially tangential to the bore 11. The medial regions of the conductor 19 engage the coating as at 20 to establish the electrical connection. The ends of the conductor 19 make electrical Contact with a terminal .or attachment ring 2,1 which surrounds the tube 10. The terminal may be secured to conductor 19 by solder. The lead wire 17 may be soldered to the ring 21 completing the upper lead-in connection to the coating.

-The lower lead-in yconnection is the same as the upper connection, and, for descriptive purposes, it is deemed suificient to apply the reference numerals 22 and 23 to the conductor `and ring, respectively, employed in making the electrical connection.

No electrical circuit for the present variable resistancev 'height of the mercury column will short out an increment of the ohmic resistance of the coating 16 so that the total resistance'of the electrical circuit will be decreased.'

Finally, when the mercury column reaches the level of the top lead-in wire 17, the only electrical resistance between the two terminal rings 21-and 23 will be the ohmic resistance of the mercury column itself. 'This value is sumciently low as to be practically negligible, and the two terminals may be regarded as being electrically shorted. At this mercury column level the electric circuit will attain its minimum ohmic resistance value for maximum current ow. Y

The invention is not to be limited to thermostatic applications alone nor is it to be limited to the exact arrangement of parts shown in the accompanying drawing or described in this specification as various changes in the details of construction may be resorted to without departing from the spirit of the invention. For example, the number of lead-in wires (17 and 18) illustrated herein is purely exemplary as is the means shown for electrically connecting the lead-in wires to the coating 16. Only insofar as the invention has particularly been pointed out in the accompanying claims is the same to be limited.

I claim:

l. A variable electrical resistance device of the liquid column type including a glass capillary tube having a central bore hermetically sealed at opposite ends; a resistance element comprising a tubular lining having a wall thickness 0f approximately .000002 inch bonded to the wall of the bore of said capillary tube and composed of a solid solution of a metallic carbide in a metal selected from a group of carbide in metal solutions consisting of molybdenum carbide in molybdenumrand tungsten carbide in tungsten which solid solution has a coeicient of expansion substantially identical to the coeicient of expansion of said capillary tube; electrodes sealed through the wall of said capillary tube at spaced locations lengthwise thereof for establishing electrical Vcommunication with said resistance element; and an electrically conductive liquid contained within said tube and movable as a column within said resistance element in response to changing physical conditions to short out portions of the resistance element.

2. A variable resistance device as deiined in claim l characterized in that the overall ohmic value of said resistance element is relatively high by comparison to the ohmic value of a portion of the liquid column equal in length and diameter to the length and diameter respectively of the central bore of said resistance element.

References Cited in the tile of this patent UNITED STATES PATENTS Hudson a May 29, 1956 

